Water filtration systems can include pour-through pitcher systems, countertop systems, and filtration cartridge systems. Pour-through pitcher systems can include an upper reservoir for receiving unfiltered water, a lower reservoir for receiving and storing filtered water, and a filtration cartridge with an inlet at its top and outlet at its bottom, through which water flows from the upper reservoir is filtered and travels to the lower reservoir.
Countertop or standalone systems can include a larger filtered water tank with a spigot for draining filtered water into a glass or other container. Pitcher and countertop systems use gravity to move the unfiltered water in the top reservoir through a water filtration cartridge and into the lower reservoir where the filtered water is stored until it is used.
Water filtration cartridges often used in gravity flow systems often include a sieve system and a housing. The housing is filled with filtration media, and the sieve system is sealed to the barrel. The filtration media is usually granular, such as activated carbon. Other components can be combined to enhance water filtration capability, such as ion exchange resin, zeolite, ceramics, and the like.
A problem associated with using granular filtration media in gravity flow cartridges is that air gets trapped in-between the particles of filtration media and in the headspace of the cartridge housing. Air can enter into the cavities of the water flow path when the filter is not used for a period of time. When the filter is again used to filter the liquid, the heavier liquid traps the lighter air in the cavities of the filter, especially close to the underside of the filtration media. This “headspace” area between the fill line of the filtration media and the underside of the filtration media provides a back pressure and a loss of filtration surface area. Both “air lock” effects contribute to the reduction in the flow rate of the liquid through the filter. This air must be vented or otherwise removed from the cartridge as water is filtered or water flow will be slow or completely stop.